1. Field of the Invention
This invention relates to an electromagnetic switch for opening and closing a circuit such as an excitation coil of an electromagnetic switch of a starter motor of an internal combustion engine.
2. Discussion of Background
Electromagnetic switches are employed in opening and closing an excitation circuit of an electromagnetic switch of a starter motor, a glow relay of a heater circuit of a glow plug of a Diesel car or the like.
FIG. 5A is a longitudinal sectional diagram of a conventional electromagnetic switch disclosed for instance, in Japanese Utility Model Publication No. 30770/1989. In FIG. 5A, a reference numeral 1 designates a yoke composing a case, 2, an excitation coil wound around a bobbin made of a synthetic resin material, and 4, a disk body made of a magnetic material contacting an end portion of the yoke 1, composing a fixed core 6 along with a cylindrical body 5 made of a magnetic material and fixed to an inner peripheral portion thereof. A numeral 7 designates a magnetic path plate made of a magnetic material and fixed between other end portion of the yoke 1 and a flange 3a on the other side the bobbin 3, and 8, a bush made of a nonmagnetic material fitted into an inner periphery of the bobbin 3, at an outer end portion of which three protrusions 8a protruding inwardly are formed by pressing. A numeral 9 designates a movable core movably supported by an inner periphery of the bush 8 in the axial direction, forming a plunger, which usually is received by the protrusions 8a in retracting thereof. A numeral 10 designates a movable terminal axle made of a synthetic resin material, at which front end side a movable terminal 11 is fitted and which is received by a stop ring 12 and is pressed by a terminal pressing spring 13. A numeral 14 designates a spring for retracting which retracts the movable terminal axle 10 and the movable core 9 contacting thereto. A numeral 15 designates an insulation cap made of a synthetic resin material covering up the terminal portion, which is attached to an end portion of the yoke 1 by screws (not shown) along with a packing 16 made of a gummy material and the disk body 4. A numeral 17 designates a pair of fixed terminals fixed to the insulation cap 15, opposing the movable terminal 11. A terminal bolt 18 which is integrally formed with the fixed terminal is outwardly extended therefrom. A numeral 19 designates a terminal connected with a lead wire 2a from the excitation coil and 20, an attaching band.
FIG. 5B is a sectional diagram taken alone a B.sub.5 --B.sub.5 of FIG. 5A. The magnetic path plate 7 is fixed to the outer periphery of the bush 8 and the retracted movable core 9 is received by the protrusions 8a of the bush 8.
Next, explanation will be given to the operation. When current flows in the excitation coil 2, the movable core 9 is sucked by the fixed core 6. The movable terminal axle 10 is forwarded thereby, the movable terminal 11 closes the fixed terminals 17 and a circuit connected to the terminal bolt 18 is closed.
In the conventional electromagnetic switch, the retracted movable core 9 is received by the protrusions 8a of the bush 8 by contacting thereto, and a metallic impact sound is generated which is offensive to ears. Furthermore, the movable core 9 is pressingly received by internal ends of the thin-walled protrusions 8a in a small contact area, wear on both contact surfaces thereof is promoted by a repetitive action thereof and the function thereof is hampered. Furthermore, with respect to the forming of a plurality of protrusions 8a on the thin-walled cylindrical bush 8 by pressing, the fabrication thereof is difficult.